1. Field of the Invention
The present invention relates to a night vision device. More particularly, the present invention relates to a night vision device of the type which removably and relatively movably attaches to a support structure which is secured to a face mask or to a helmet, for example, to thereby be supported in front of at least one eye of the user. As thus supported, the night vision device may be used by the wearer of the helmet or face mask to view a night scene while the user's hands remain free. Still more particularly, the present invention relates to such a night vision device having a significantly reduced weight and polar moment of inertia in comparison to a conventional night vision device. Thus, the inventive night vision device significantly reduces the weight carried on the head of a user of the device and reduces neck strain for this user, as well as improving the ease of use of the night vision device by improving its maneuverability.
2. Related Technology
A conventional night vision device is known as the AN/PVS-7B. This night vision device includes a housing with a single objective lens through which low-level light from a night scene is received. The night scene may be illuminated, for example, only by star light. The housing of the conventional AN/PVS-7B also included an image intensifier tube which provides a phosphor yellow-green light image representative of the night scene of low-level illumination viewed through the single objective lens, an image splitter, and a pair of eye pieces in which a user of the device may view the intensified image with each eye.
As will be well understood to those ordinarily skilled in the pertinent arts, the head-mounted night vision devices all represent a certain weight which the users of the devices must carry on their heads. Additionally, this weight for many night vision devices, such as the AN/PVS-7B, is cantilevered forwardly of the user's face. Consequently, the user experiences a significant downward weight tending to nod his head forward. In order to resist this nodding moment on his head, the user must make a conscious effort to keep his head up. After a period of wearing the night vision systems, users become accustomed to the extra effort necessary to keep their heads level. However, complaints of neck fatigue and strain are common.
Additionally, the cantilever mounting of such night vision devices places their mass at a considerable distance from the pivot axis for the user's head when the head is turned from side to side. Generally, this pivot axis may be considered to be at the vertical alignment of the user's spine. Accordingly, the mass of the night vision device is several inches from this pivot axis, and represents a significant moment of inertia which the user must overcome in turning his head. Also, when the user's head is turned quickly and stopped, the inertia of the night vision device tends to make the device overshoot the user's head position, and to carry the user's head along in this overshoot.
One partial, and not completely satisfactory, solution to this nodding weight problem is to counterbalance the weight of the night vision device. Accordingly, some night vision systems simply hang a battery pack for the device off the back of the helmet which the user is wearing. However, this expedient further increases the weight which the user must carry on his head, and very significantly increases the inertia of the night vision system. That is, the moment of inertia of the night vision system varies as the integral of the masses of the system components times the square of the radius of the masses from the pivot axis. Thus, its is easy to see that placing a relatively heavy battery pack at the rear of the user's head adds considerably to the inertia of the complete night vision system.